In order to have poison free aquarium water, filtration process is a very important to aquarist. Since most aquarists are aware of the fact that their aquarium water gradually accumulates potentially harmful substances, which eventually poison their fishes. In order to prevent this from happening there is need for filtration therefore, for our purpose I would define filtration as the removal of unwanted substances from water.
Most aquarist uses three types: Biological, mechanical, and chemical amongst which the biological is the most important.
Biological filtration is also referred to as undergravel filtration, because the major equipment used (a flat plate of perforated plastic) is placed under the gravel bed hence it is invisible. Unless you are properly equipped, you can’t see the process happening and cannot measure its effect.

Yet this process is the major difference between success and failure, and the aquarist who does not take the time to understand it workings is doomed to watch an endless procession of dying fishes passing through his tank.
Biological filtration is solely the work of bacteria attached to the surfaces of the gravel, the grave and the under grave filter together constitute the filter bed. Bacteria normally reach the filter bed through the food you give the fishes.
The waste product of the fishes, the air, and even through your hands as you works in the tank.
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